An EGR apparatus is known which recirculates part of exhaust gas from an engine in a vehicle or the like to the engine to suppress generation of nitrogen oxides. Some of such EGR apparatuses are equipped with, midway of an exhaust gas recirculation line to the engine, an EGR cooler for cooling the exhaust gas since cooling the exhaust gas to be recirculated to the engine will drop the temperature of and reduce the volume of the exhaust gas to lower the combustion temperature in the engine without substantial decrease of output of the engine, thereby effectively suppressing generation of nitrogen oxides. Known in this regard is, for example, JP 2001-74380 A.
FIGS. 1 and 2 are sectional views showing a first example of the above-mentioned EGR cooler in which reference numeral 1 denotes a cylindrical shell with axial opposite ends to which plates 2 are respectively fixed to close the ends of the shell 1. A number of tubes 3 extend axially within the shell 1 and are penetratingly fixed at their opposite ends to the respective plates 2.
Cooling water inlet and outlet 4 and 5 are attached to the shell 1 near one and the other end thereof, respectively, so that cooling water 9 is supplied via the inlet 4 into the shell 1, flows outside of the tubes 3 and is discharged out of the shell 1 from the outlet 5.
The respective plates 2 have, on their sides away from the shell 1, bowl-shaped hoods 6 fixed to the plates 2 so as to enclose end faces of the plates 2. The one and the other hoods 6 provide central gas inlet and outlet 7 and 8, respectively, so that the exhaust gas 10 from the engine enters via the gas inlet 7 into the one hood 6, is cooled during passage through the tubes 3 by heat exchange with the cooling water 9 flowing outside of the tubes 3 and is discharged from the gas outlet 8 to the other hood 6 to be recirculated to the engine. In FIG. 1, reference letter x denotes axial extension line for the shell 1.
However, such EGR cooler shown in the first example is disadvantageous in that the cooling water 9 supplied via the inlet 4 into the shell 1 nonuniformly flows to the outlet 5 with respect to internal cross section of the shell 1, so that the cooling water 9 may stagnate near corners on opposite sides of the inlet 4 and outlet 5 in the shell 1 as shown by a course 11 to generate cooling water stagnation zones 12, resulting in localized high temperature of and thus thermal deformation of the tubes 3 near the stagnation zones 12.
To overcome this, a second example of an EGR cooler as shown in FIG. 3 is provided which has a bypass conduit 14 extending outside from the shell 1, at a position diametrically opposite to the inlet 4, to the outlet 5. The conduit 14 drains part of the cooling water 9 having been introduced via the inlet 4 so as not to cause stagnation of the cooling water 9 and to prevent the stagnation zone 12 from being generated at the diametrically opposite position to the inlet 4, thereby suppressing any localized high temperature of the tubes 3.
However, in the second example of the EGR cooler, arrangement of the bypass conduit 14 outside of the shell 1 is disadvantageous in that it interferes with peripheral devices of the shell 1 to substantially lower mountability onto a vehicle.
Thus, the invention was made in view of the above and has its object to provide an EGR cooler which prevents generation of cooling-water stagnation zones and improves its mountability onto a vehicle.
Conventional EGR coolers include a third example. Known as such is, for example, JP 2000-213424 A.
FIG. 4 is a sectional view showing the third example of an EGR cooler in which reference numeral 31 denotes a cylindrical shell with axially opposite ends to which plates 32 are respectively fixed to close the ends of the shell 31. A number of tubes 33 substantially of the same diameter extend axially within the shell 31 and are penetratingly fixed at their opposite ends to the respective plates 32.
Cooling water inlet and outlet 34 and 35 are fixed from outside to the shell 31 near one and the other ends thereof, respectively, so that cooling water 39 is supplied via the inlet 34 into the shell 31 to flow outside of the tubes 33 and is discharged via the outlet 35 outside of the shell 31.
The respective plates 32 have, on their sides away from the shell 31, bowl-shaped hoods 36 fixed to the plates 32 so as to enclose end faces thereof. The one and the other hoods 36 provide central exhaust-gas inlet and outlet 37 and 38, respectively, so that exhaust gas 40 from the engine enters via the inlet 37 into the one hood 36, is cooled during passage through the tubes 33, by heat exchange with the cooling water 39 flowing outside of the tubes 33 and is discharged via the outlet 38 to the other hood 36 to be recirculated to the engine.
In the figure, reference numeral 41 denotes a bypass outlet conduit which is arranged on the shell 31 at a position diametrically opposite to the inlet conduit 34 and which drains part of the cooling water 39 to prevent stagnation of the cooling water 39 at the position opposite to the inlet conduit 34.
The tubes 33 are arranged as shown in FIG. 5 such that the plural tubes 33 with the same diameter are at a constant pitch in the form of concentric circles about the axis O the shell 31, the peripheral tubes 33 being positioned along the shell 1 while a central tube 33a is positioned at an axis O of the shell 31.
However, even with the third example of the EGR cooler having the tubes 33 arranged at the constant pitch in the form of concentric circles about the axis O of the shell 31, the hot exhaust gas 40 via the inlet 37 tends to flow more through the tubes 33 on the center side than through the tubes on the peripheral side, so that the tubes 33 on the center side may become higher temperature than those on the peripheral side, resulting in possibility of localized thermal deformation and deteriorated heat exchange efficiency.
Thus, the invention was made in view of the above and has its object to provide an EGR cooler in which tubes on the center side may be arranged to be effectively cooled.